HYALONEMA (OONEMA) SEQUOIA. 
357 
More strongly aberrant forms are much rarer. A small macramphidisc of this 
kind (Plate 89 , figs. 35, 36) is 112 /x long, and has a shaft 14 n thick. The termi- 
nal anchors are very irregular, spirally twisted, and on one side much longer than 
on the other. The chords of the longest anchor-teeth are more than three 
quarters of the whole spicule in length. 
The length frequency-curve of the large micramphidiscs also has two 
summits, but as the depression separating them is slight, and as the spicules to 
which the two parts of the curve on the two sides of it refer, are very similar in 
shape, I do not consider this irregularity of the length frequency-curve sufficient 
for dividing the large micramphidiscs into two groups. 
The large micramphidiscs (Plate 89 , figs. 6-14) are 63-122 n long, most 
frequently about 93 /x. The shaft is 2.5-4 /x thick, and generally thickened in its 
middle-part to a central tyle 5-6 m in diameter. Rarely it is of uniform thick- 
ness throughout and without a tyle. With the exception of its end-parts, which 
are smooth, the whole of the shaft is covered with spines. The spines arising 
from the tyle are usually arranged in an irregular oblique verticil. These spines 
are larger than the others. The terminal anchors are 29-43 /x long and 25-40 n 
broad. The proportion of their length to their breadth is 100 : 75 to 100 : 105, 
on an average 100 : 87.9. The proportion of the anchor-length to the length of 
the whole spicule is 1 to 2.4-3. 6, on an average 1 : 2.7. The individual anchor- 
teeth are curved strongly at the base, but curved only slightly in their middle- 
part. The tip of the tooth is frequently abruptly bent inwards. Apart from 
this abruptly bent end-part, the distal half of the tooth diverges from the shaft 
at an angle of 6°-12°. 
I found an abnormal large micramphidisc with strongly reduced terminal 
anchors. This spicule (Plate 89 , figs. 16-19) is 100 n long. Its shaft is straight, 
8 /x thick, and covered with numerous scattered tubercles and a verticil of short, 
stout, cylindrical, terminally rounded spines. The terminal anchors are rudi- 
mentary, only 17 /x long and 23 /x broad, and composed of a terminal tyle enclosed 
by thin leaf-like teeth, most of which terminate with two terminal spines. 
The length frequency-curve of the small micramphidiscs has three sum- 
mits. The two depressions separating them are inconsiderable, and the small 
micramphidiscs of different sizes differ only in that the smallest generally have 
a smooth shaft, the larger generally a spiny one. Although the smallest of these 
amphidiscs, belonging to the elevation of the curve to the extreme left, might 
therefore be separated from the others, I think it best to consider them all as 
forming a single group, and describe them together. 
